ipdivert: check that PCB is still valid after taking INPCB_RLOCK.
We are inspecting PCBs of divert sockets under NET_EPOCH section,
but PCB could be already detached and we should check INP_FREED flag
when we took INP_RLOCK.
PR: 254478
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29420